1. Field of the Invention
The present invention relates to a liquid crystal display device (liquid crystal display) and, more specifically, to improving contrast ratios of the liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices (liquid crystal display) exhibit a characteristic of being capable of implementing high definition with low power consumption, and those display devices are employed broadly to various devices from small portable telephones to large television monitors.
While the liquid crystal displays are used broadly, there is an issue in terms of the contrast ratios. The contrast ratio of the liquid crystal display in a dark place is normally about 1000:1. This is inferior to that of discharge-type displays such as CRT, plasma display, and FED/SED. This fails to provide a sense of lively reality, when displaying an image source such as a movie with a full expression in black sections.
There is Japanese Unexamined Patent Publication S64-010223 (Patent Document 1), for example, as a technique for overcoming such issue. FIG. 39 is an explanatory illustration showing a structure of a liquid crystal display panel 900 according to Patent Document 1. This liquid crystal display panel 900 includes a plurality of TN-type liquid crystal display panels 941 and 942 which are substantially in a same shape. Each of the liquid crystal display panels 941 and 942 is formed with a pair of transparent substrates 911, 912 and a pair of transparent substrates 913, 914 having electrodes for driving liquid crystals, TN-type liquid crystal layers 931, 932 interposed between the pair of transparent substrates, and polarization plates 901, 902, 903, 904 disposed on both sides of the pair of transparent substrates. Each of corresponding electrodes 921-924 are stacked to be superimposed one on another completely in the direction of an optical axis 951, and each of the liquid crystal display panels 941 and 942 are driven simultaneously with a same drive signal.
Through employing such structure, it is possible to improve the contrast ratio that is about 10-15 with a structure using a single liquid crystal display panel to about 100:1 by stacking two panels, when the contrast ratio is measured by using a laser beam. Further, the contrast ratio can be improved to about 1000:1 by stacking three panels. It is so described in Patent Document 1 that such structure can achieve the contrast ratio that is above the limit of the contrast that can be displayed with a single liquid crystal display panel.
Further, Japanese Unexamined Patent Publication 2007-286413 (Patent Document 2) discloses a technique which decreases color changes generated due to a viewing direction by providing a color filter to one of n-pieces of stacked liquid crystal display panels, considering the fact that light passes through different-color layers of color filter layers and the colors are mixed on a lower-layer liquid crystal display panel and an upper-layer liquid crystal display panel when a viewer moves the visual field physically in a case where the liquid crystal display panels having color filter layers are stacked. This technique further applies averaging processing on a video source by corresponding to a single to (n−1)-pieces of liquid crystal display panels among the n-pieces of stacked liquid crystal display panels so as to decrease a parallax generated due to the viewing directions while improving the contrast ratio of the liquid crystal display device greatly.
Further, Japanese Unexamined Patent Publication 2008-122940 (Patent Document 3) discloses a technique which keeps luminance of regions with luminance 100 in bright-point display having the bright luminance 100, and applies averaging processing on a luminance distribution where there is a change in the brightness (light and dark) in a part with luminance 0 in the boundary sections between pixels of luminance 100 and pixels of luminance 0. Japanese Unexamined Patent Publication H11-015012 (Patent Document 4) discloses a display device having three or more stacked liquid crystal display panels, in which the liquid crystal display panel as an intermediate layer is thinner than the other layers, and a nonlinear element is provided to one of two substrates sandwiching the intermediate layer.
It is true that Patent Document 1 is capable of improving the contrast ratio through driving the two stacked liquid crystal display panels by same signals from a same signal source. However, the liquid crystal layers are isolated by a specific distance in the thickness direction. Thus, when a viewer moves the visual field physically, position shift (parallax) in the displays occurs between those liquid crystal layers depending on the angles (viewing directions). This raises another issue such as deterioration in the visibility.
In the meantime, the technique depicted in Patent Document 2 is capable of displaying videos with a sense of lively reality since it can provide a full expression in black sections when displaying videos of relatively moderate luminance changes, such as images of nature displayed often on TVs and movies.
However, the technique of Patent Document 2 applies averaging means to the video source, so that images generated thereby come to have a dull difference between the levels of the luminance. Thus, transmittance is deteriorated in the liquid crystal elements of one to (n−1)-pieces of liquid crystal panels among the n-pieces of stacked liquid crystal display panels. Therefore, display luminance of such liquid crystal display device in which the panels are stacked comes to be deteriorated when displaying videos of sharp luminance changes, such as text display and fine pattern display.
Further, the technique of Patent Document 3 can keep the luminance of the one to (n−1)-pieces of liquid crystal display panels among the n-pieces of stacked liquid crystal display panels in the regions located in the perpendicular direction of the panels of the pixels regions of luminance 100, so that luminance deterioration does not occur in the front visual field. Therefore, the technique of Patent Document 3 can be considered effective in this respect.
However, with the technique of Patent Document 3, position shift occurs in the n-pieces of stacked liquid crystal display panels depending on the viewing directions, and the luminance in that region is deteriorated since the luminance of the one to (n−1)-pieces of liquid crystal display panels among the n-pieces of stacked liquid crystal display panels is attenuated. Further, with the technique of Patent Document 3, there still remains an issue of having color changes since the ratio of light amount passing through each dot of the liquid crystal display panels having the color filter layer changes. The technique of Patent Document 4 is designed to overcome generation of parallax through forming the liquid crystal display panel as the intermediate layer to be thinner than the other layers. However, the distance in the thickness direction is not reduced, so that it is not possible to overcome the issues of luminance deterioration and color changes.